In view of their exceptional stability, resistance to abrasion and extensive handling stresses, and stability under varying atmospheric conditions, polyester film based drafting sheet material has found extensive acceptance in the field of technical and engineering drawing.
Such drafting films consist essentially of a substantially transparent film of a polyester, such as polyethylene terephthalate, having a coating thereon of an adherent matrix of polymeric composition having dispersed throughout its thickness sufficient finely-divided pigment to impart a surface quality of ink- and pencil-receptiveness, as well as to yield a sufficient opacity or translucency to accord acceptable legibility during use.
The manufacture of such drafting film products has hitherto entailed the application by suitable coating means of a uniform layer of a fluid composition comprising a dispersion or slurry of pigment in a vehicle comprising a solution, most often in organic solvent, of selected polymeric compounds or blends thereof. Subsequent to such application of coating composition, circulating heated air has been utilized to remove the volatile solvents as vapor, resulting in solidification of the coated layer as a pigmented film adherent to the polyester base. Representative of such earlier drafting film products and the coating procedures involved in their manufacture are those discussed in U.S. Pat. Nos. 2,999,016 and 3,100,722.
Attendant to the manufacture of drafting film material by means utilizing volatile vehicle coating technology has been the uneconomical and, when utilizing organic solvent, dangerous drying procedures, and more recently the obtrusive problem of unrecovered vapor wastes disposal. Manufacture of the earlier products has also suffered from limitations dictated by the susceptibility of base sheet or coating composition to otherwise effective processing conditions such as drying temperature, massive air volume handling, and the like.